(a) Field of the Invention
The present invention relates to a zoom lens system for cameras, and more specifically for video cameras.
(b) Description of the Prior Art
In recent years when compact and light-weight video cameras have been offered at low prices, there are increasing demands for domestic video cameras. In order to further expand these demands, it is necessary to develop more compact video cameras which are lighter in weight and marketable at lower prices, thereby posing problems to design lens systems which are more compact in external designs, lighter in weight and manufacturable at lower costs.
Under the present circumstances, zoom lens systems for domestic video cameras are designed mostly for a zooming ratio of 6 and aperture ratios of F/1.2 to F/1.6 because these specifications are convenient from viewpoints of both lens design and practical use. Each of these zoom lens systems for video cameras generally consists of four lens units as exemplified by Japanese Unexamined Published Patent Applications No. 102208/58 and No. 153913/58. However, each of these zoom lens systems has defects. For example the lens system comprises lens elements numbering as large as 14 to 15, the lens system requires a high manufacturing cost, the lens system has large external dimensions and the lens system is heavy in weight. Further, the zoom lens system disclosed by Japanese Unexamined Published Patent Application No. 110112/61 comprises lens elements in a reduced number of 8 owing to a large number of aspherical surfaces used therein, but requires a high manufacturing cost due to an extremely short radius of curvature adopted on a cemented surface in a cemented doublet arranged therein.
Further, the zoom lens systems each consisting of four lens units are of a type which can hardly have large field angles since diameters of the front lenses thereof are rapidly prolonged when the field angles of such zoom lens system exceed 50.degree. at the wide positions thereof. The zoom lens systems which are designed for high vari-focal ratios and field angles far larger than 50.degree. at the wide positions thereof have common defects that the front lenses thereof have large diameters and that the lens systems can hardly correct aberrations with a small number of lens elements. In order to correct the aberrations, these zoom lens systems must comprise a large number of lens elements and move a large number of lens units, thereby complicating compositions and requiring high manufacturing costs.
Accordingly, attention is paid to the conventionally known zoom lens system which consists of a front lens unit having negative refractive power and a rear lens unit having positive refractive power. This zoom lens system does not require remarkably prolonging the diameter of the front lens thereof even when field angle at the wide position thereof exceeds 50.degree., comprises a small number of lens elements, six to eight, and has a simple composition for moving two lens units only. However, this zoom lens system can hardly have a high zooming ratio nor a large aperture ratio.
Furthermore, a stop is generally arranged in the rear lens unit and moved together with the rear lens unit for varying focal length. When the above-described zoom lens system consisting of two lens units is used in a video camera comprising a large and heavy mechanism for autoiris, a complicated composition is required for moving such a heavy stop along the optical axis.
As a conventional example capable of correcting this defect, there is known the zoom lens system disclosed by Japanese Unexamined Published Patent Application No. 266511/62. This patent application discloses only the zoom lens system which consists of two lens units and comprises a stop fixed between the front lens unit and the rear lens unit, but makes no disclosure of numeric data on an actual zoom lens system. In addition, the zoom lens system disclosed by this patent application is undesirable for use with a video camera since the exit pupil is located in the vicinity of the image surface especially at the telex position of the zoom lens system.